It may be that you have some muscular irritation although this would generally have pain with movement or pressure.
If your pain is worse with deep breaths it may be pleurisy, a condition where the lining of the lungs is inflamed.
A less likely possibility is an infection in the breast. The location is a bit unusual for this.
I would recommend you have this evaluated if it does not begin to improve soon. You can try some ibuprofen to see if this helps. Ibuprofen is safe during breast feeding for short durations.
Advice and opinions given are based on the information provided and must not be considered an official medical recommendation. The limitations of the internet prevent an appropriate evaluation. Always consult your doctor if there is concern.